I found an install.log file under (Build directory)\x64\Release\INSTALL that said
Cannot access “C:\Program Files (x86)\Blender\blender.crt\msvcp140.dll” Permission denied.
Unfortunately, it’s not as simple as R-clicking the .sln and “run as admin”. However, can run Visual studio 2019 as admin and load “solution”. (I hadn’t tried this before because Visual Studio layout if different with the solution loaded. I was thinking the build had custom built portions of Studio as well.)
Trying now, will reply if works. 